Phytochrome‐mediated Phototropism in Adiantum Protonemata II. Participation of Phytochrome Dark Reversion

A microbeam irradiation technique was used to analyze phytochrome‐mediated phototropism of the protonema of the fern Adiantum capillus‐veneris. One side of the sub‐apical zone of a dark‐adapted protonema was irradiated with a red‐light (R) microbeam to induce phototropic curvature toward the irradiated side. Except the cases where fluence‐response relationships were examined, the protonema was stimulated with the microbeam for about 30 s to provide a fluence (5.3–5.5 mmol m ‐2 ) optimal for curvature. When the whole protonema was pretreated with a high fluence of R (about 9 mmol m ‐2 ), no significant curvature could be induced by immediately subsequent one‐sided microbeam stimulation. It was found, however, that curvature became inducible progressively as the time of microbeam stimulation was delayed after the R pretreatment. The protonema gained nearly full responsiveness within 40 min after the pretreatment. Moreover, the inductive effect of microbeam R escaped rapidly from the reversing effect of far‐red light. These results indicated that most of the far‐red‐absorbing form of phytochrome (Pfr), which mediates the phototropic response, undergoes relatively fast dark reversion to the red‐absorbing form. The dark reversion kinetics was analyzed further by taking into account that the phototropic responsiveness after the R pretreatment was measured in the presence of background Pfr over the protonema. This analysis revealed a rate constant of about 0.002 s ‐1 (t 1/2 ˜ 6 min) for the dark reversion. It is considered that the Pfr dark reversion plays a role in establishing a lateral Pfr gradient in the unilaterally irradiated protonema.